An apparatus for capturing co 2 from gas containing co 2 and method of operating the same

Abstract

Invention relates an apparatus for capturing CO2 from gas containing CO2, comprising a calcium looping of fluidized bed carbonator reactor (100), and a fluidized bed calciner reactor (200). A flow path of solid material extending between the carbonator reactor (100), and the calciner reactor (200) comprises a fluidized bed mixing chamber (300), which dampens fluctuation of CaCO3 in the solid material flow fed to the calciner reactor (200). Invention relates also to a method of operating the apparatus.

Description

An apparatus for capturing CO2from gas containing CO2and method of operating the sameTechnical field

[0001] The present invention relates to an apparatus for capturing CO2from gas containing CO2.

[0002] The present invention relates to a method of operating an apparatus for capturing CO2from gas containing CO2.Background art

[0003] There are many industrial processes, such as combustion of carbonaceous fuel and cement clinker production, which result in carbon dioxide containing gas. Calcium looping (CaL) is a known carbon capture technology in abatement of CO2emission.

[0004] Chemical reaction to which calcium looping is based is as follows:CaCO3« — CaO + CO2, AH = + / - 178 kJ / mol

[0005] There are two main processes in CaL: In the 'forward' reaction in the equation above calcination takes place where solid calcium carbonate is fed into a calciner, where it is heated to 850-950 °C to cause it to thermally decompose into gaseous carbon dioxide and solid calcium oxide (CaO). The heat required for this reaction is provided by burning a fuel with a mixture of pure oxygen and recirculated CO2-rich calciner flue gas, also known as oxy-fuel combustion. Moreover, the supply of heat to the calciner could be arranged by other means, including for instance electrification. The almost pure stream of CO2is then removed and purified so that it is suitable for storage or use.

[0006] In the 'backward' reaction in the equation above carbonation takes place where the solid CaO is removed from the calciner reactor and fed into the carbonator reactor. It is cooled and is brought into contact with a gas containing CO2. The CaO and CO2react to form CaCO3, thus reducing the CO2concentration in the gas to a level suitable for emission to the atmosphere.

[0007] Calcium looping is generally practiced by making use a dual Fluidized Bed (FB) unit configuration. The flue gas from an emission source is arranged in contact with the sorbent, calcium oxide, in the first fluidized bed (carbonator), where it mineralizes into calcium carbonate and the flue gas exits clean of CO2from the unit. The calcium carbonate is then directed to an oxyfuel fluidized bed calciner, in which it is regenerated back to calcium oxide, in the presence of fuel and oxygen.

[0008] Often the emission source produces gaseous CO2in non-constant manner, such that the flow rate of CO2containing exhaust gas fluctuates and / or concentration of CO2in the exhaust gas fluctuates, but still the plant performance needs to be dimensioned based on maximum required capacity.

[0009] An object of the invention is to provide an apparatus for capturing CO2and method of capturing CO2from gas containing CO2in which the performance is considerably improved compared to the prior art solutions.Disclosure of the Invention

[0010] Objects of the invention can be met substantially as is disclosed in the independent claims and in the other claims describing more details of different embodiments of the invention.

[0011] An apparatus for capturing CO2from gas containing CO2according to the invention comprises a fluidized bed carbonator reactor and a fluidized bed calciner reactor arranged to cooperate with each other, wherein a. the fluidized bed carbonator reactor comprising i. a carbonator chamber ii. a first inlet for CO2containing gas in the carbonator chamber at bottom of the fluidized bed carbonator reactor,Hi. a second inlet for solid material in the carbonator chamber, iv. a first outlet for gas-solid material suspension in the carbonator chamber, v. at least one solid material separator, having:1. an inlet for gas-solid material suspension connected with the first outlet for gas-solid material suspension,2. a first outlet for separated solid material, and3. a second outlet for exhaust gas of reduced CO2concentration, vi. a first solid material return system for separated solid material extending from the first outlet for separated solid material to the carbonator chamber, b. the fluidized bed calciner reactor comprising i. a calciner chamber ii. a first inlet for solid material in the calciner chamber,Hi. a second inlet for fuel material in the calciner chamber, iv. a third inlet for oxygen containing gas in the calciner chamber, v. a first outlet for gas-solid material suspension in the calciner chamber. vi. at least one solid material separator, having:1. an inlet for gas-solid material suspension connected with the fist outlet for gas-solid material suspension,2. a first outlet for separated solid material, and3. a second outlet for CO2enriched gas, vii. a second solid material return system for separated solid material extending from the first outlet for separated solid material to the calciner chamber, and the apparatus further comprising c. a first flow path for solid material extending between the fluidized bed carbonator reactor and the calciner chamber of the fluidized bed calciner reactor, d. a second flow path for solid material extending between the fluidized bed calciner reactor and the carbonator chamber of the fluidized bed carbonator reactor, wherein e. the first flow path comprises a fluidized bed mixing chamber, comprising i. inlet for solid material arranged in flow connection with the carbonator reactor, ii. inlet for fluidization gas,Hi. outlet for solid material arranged in flow connection with the calciner reactor, iv. outlet for fluidization gas.

[0012] When the fluidized bed mixing chamber is arranged to the first flow path for solid material extending between the fluidized bed carbonator reactor and the calciner chamber of the fluidized bed calciner reactor, it is possible to even out transient fluctuation of CaCO3concentration in a stream of solid material which is transferred from the carbonator reactor to the calciner reactor. This in turn influences required capacity of the calciner reactor. Namely, in practise the calciner reactor needs to be dimensioned to be able to regenerate the reactant material for carbonation in each circumstance, and therefore the calciner reactor needs to be dimensioned based on the peak concentrations of CaCO3in the flow of solid material, i.e. kind of over dimensioned to have excessive capacity. Solid material which is fed from the carbonator to the fluidized bed mixing chamber, is fluidized in the mixing chamber and mixed therein over a predetermined period of time, a residence time, which results in equalizing CaCO3concentration in an output stream of solid material from the fluidized bed mixing chamber. This way even if there is fluctuation of CaCO3concentration in the stream of solid material which is extracted from the carbonator reactor, CaCO3concentration in the stream of solid material which is fed to the calciner reactor is substantially even. Thus, with in the apparatus according to the invention the fluidized bed calciner reactor does not need to be over dimensioned. In this context solid material refers to material containing all solid components in the material, including the reactant material, which particularly refers to CaCO3and CaO.

[0013] According to an aspect of the invention the outlet for fluidization gas in the fluidized bed mixing chamber is arranged in flow connection with the carbonator reactor. The carbonator is already treating exhaust gas and therefore leading the fluidization gas to carbonator reactor provides more freedom to select the fluidization gas. Preferably the outlet for fluidization gas is connected to the fluidized bed carbonator chamber, but in some practical application the connection may be with gas outlet of solid material separator.

[0014] According to an aspect of the invention the second flow path consists of a solid material transfer channel extending from the second solid material return system to the carbonator reactor. The second flow path can be made as a simple transfer channel since the fluidized bed mixing chamber in the first flow path provides adequate buffer for solid material.

[0015] According to an aspect of the invention the first flow path is cooler-free, particularly the fluidized bed mixing chamber comprises a mixing space of the fluidized bed mixing chamber, which mixing space is a cooler-free space. This refers cooler of solid material. By providing a cooler-free space decrease of temperature of material in the mixing chamber is minimized.

[0016] According to a further aspect of the invention the first flow path, preferably the mixing chamber, is provided with a heating device for preventing decreasing of temperature of the solid material. By such a heating device it can be ensured that temperature of the solid material is not decreasing during transportation from the carbonator reactor to the calciner reactor.

[0017] According to an aspect of the invention fluidized bed mixing chamber comprises substantially vertical side walls, a top wall, a wind box and a fluidization grid at its bottom part, to which wind box the inlet for fluidization gas is arranged to open, and the top wall, the side walls and the grid of the fluidized bed mixing chamber are of uncooled structure. This way no direct heat transfer is subjected from the solid material in the mixing chamber so as to provide adiabatic mixing of reactant material.

[0018] According to an aspect of the invention the top wall, the side walls of the fluidized bed mixing chamber are provided with a heat insulation. This decreases heat transfer through the wall and the adiabatic nature of mixing of the reactant material is improved.

[0019] According to an aspect of the invention the heat insulation comprises a refractory lining arranged inside the fluidized bed mixing chamber. This decreases heat transfer through the wall, improves the adiabatic nature of mixing of the solid material and provides protection of mechanical wearing to the walls of the mixing chamber.

[0020] According to an aspect of the invention the outlet for solid material comprises a vertically extending discharge channel having an opening in the lower part of the mixing chamber, wherein the outlet is arranged at an upper part of the discharge channel, wherein the discharge channel is provided with means to feed fluidizing gas to the lower part of the channel.

[0021] This makes it possible to automatically maintain the height of the fluidized bed at a desired level, since outflow of the solid material through the dischargechannel is determined by hydrostatic pressure of the bed such that when the bed height is increased, hydrostatic pressure is increased increasing solid material flow rate in the discharge channel, and when the bed height is decreased, hydrostatic pressure is decreased, which decreases solid material flow rate in the discharge channel.

[0022] A method of operating an apparatus for capturing CO2from gas containing CO2, the apparatus comprising a fluidized bed carbonator reactor and a fluidized bed calciner reactor arranged to cooperate with each other, wherein a. the fluidized bed carbonator reactor comprising i. a carbonator chamber, ii. a first inlet for CO2containing gas in the carbonator chamber at bottom of the fluidized bed carbonator reactor,Hi. a second inlet for solid material in the carbonator chamber, iv. a first outlet for gas-solid material suspension in the carbonator chamber, v. at least one solid material separator, having:1. an inlet for gas-solid material suspension connected with the first outlet for gas-solid material suspension,2. a first outlet for separated solid material, and3. a second outlet for exhaust gas of reduced CO2concentration, vi. a first solid material return system for separated solid material extending from the first outlet for separated solid material to the carbonator chamber, b. the fluidized bed calciner reactor comprising i. a calciner chamber ii. a first inlet for solid material in the calciner chamber,Hi. a second inlet for fuel material in the calciner chamber, iv. a third inlet for oxygen containing gas at bottom of the fluidized bed calciner reactor, v. a first outlet for gas-solid material suspension in the calciner chamber. vi. at least one solid material separator, having:1. an inlet for gas-solid material suspension connected with the first outlet for gas-solid material suspension,2. a first outlet for separated solid material, and3. a second outlet for CO2enriched gas, vii. a second solid material return system for separated solid material extending from the first outlet for separated solid material to the calciner chamber and the apparatus further comprising c. a first flow path for solid material extending between the fluidized bed carbonator reactor and the calciner chamber of the fluidized bed calciner reactor, d. a second flow path for solid material extending between the fluidized bed calciner reactor and the carbonator chamber of the fluidized bed carbonator reactor, wherein e. the first flow path comprises a fluidized bed mixing chamber, comprising i. inlet for solid material arranged in flow connection with the carbonator reactor, ii. inlet for fluidization gas,Hi. outlet for solid material arranged in flow connection with the calciner reactor, iv. outlet for fluidization gas. and wherein the method comprising a. introducing CO2containing gas into the carbonator chamber via the first inlet for CO2containing gas, b. feeding solid material into the carbonator chamber via the second inlet for solid material, c. arranging a fluidized bed of solid material in the carbonator chamber d. causing CO2in the gas to react with solid CaO in the fluidized bed of solid material, forming CaCO3e. removing solid material from the fluidized bed of solid material in the carbonator reactor, and feeding the removed solid material to the fluidized bed mixing chamber, f. arranging a fluidized bed of solid material in the mixing chamber by feeding fluidization gas into the mixing chamber, and mixing the incoming solid material in the fluidized bed of solid material andg- removing solid material from the fluidized bed mixing chamber and h. feeding the removed solid material to the calciner reactor.

[0023] When the fluidized bed is arranged in the mixing chamber, it is possible to even out transient fluctuation of CaCO3concentration in a stream of solid material which is transferred from the carbonator reactor to the calciner reactor. This in turn influences required capacity of the calciner reactor. Namely, in practise the calciner reactor needs to be dimensioned to be able to regenerate the solid material for carbonation in each circumstance, and therefore the calciner reactor needs to be dimensioned based on the peak concentrations of CaCO3in the solid material, i.e. kind of over dimensioned to have excessive capacity. Solid material which is fed from the carbonator to the fluidized bed mixing chamber, is fluidized in the mixing chamber and mixed therein over a predetermined period of time, a residence time, which results in equalizing CaCO3concentration in an output stream of solid material from the fluidized bed mixing chamber. This way even if there is fluctuation of CaCO3concentration in the stream of solid material which is extracted from the carbonator reactor, CaCO3concentration in the stream of solid material which is fed to the calciner reactor is substantially even. Thus, with the method according to the invention the fluidized bed calciner reactor does not need to be over dimensioned.

[0024] According to an aspect of the invention that residence time of the solid material in the fluidized bed mixing chamber is at least 200 seconds and less than 1500 seconds, preferably the residence time is 500 - 1000 seconds.

[0025] This provides preferred residence time such that the CaCO3concentration in the stream of solid material which is fed to the calciner reactor is substantially even.

[0026] According to an aspect of the invention fluidization gas is led from the mixing chamber to the carbonator reactor. This way dilution of CO2concentration of calciner outlet gas is avoided.

[0027] According to an aspect of the invention solid material is fed directly, without further treatment, from the calciner reactor into the carbonator chamber.

[0028] According to an aspect of the invention during the steps removing the solid material from the fluidized bed of solid material in the carbonator reactor , and feeding the removed solid material to the fluidized bed mixing chamber,arranging a fluidized bed of solid material in the mixing chamber by feeding fluidization gas into the mixing chamber, removing solid material from the fluidized bed mixing chamber and feeding the removed solid material to the calciner reactor, cooling of solid material is minimized.

[0029] According to more developed aspect of the invention decreasing of temperature of solid material is prevented during transportation of solid material from the carbonator to the calciner reactor, and during mixing of the solid material by providing a heating device to the first flow path, and when necessary, decrease of temperature of the solid material is prevented by bringing heat to the solid material.

[0030] According to an aspect of the invention fluidization gas fed to the mixing chamber is any gas excluding CO2. Particularly when the invention fluidization gas is led from the mixing chamber to the carbonator reactor, the fluidization gas will not unduly increase carbonation of the CaO in the carbonator reactor.

[0031] Availability, cost-effectiveness, efficient reaction properties, established industrial use, and favourable chemical characteristics of calcium make it the most practical metal for commercial carbon capture applications. These factors combined provide a robust and economically viable solution for reducing CO2emissions on an industrial scale. Generally, calcium looping is the most developed form of carbonate looping, where a metal M is reversibly reacted between its carbonate form MCO3and its oxide form MO to separate carbon dioxide from other gases coming from either power generation or an industrial plant. But still, for example metals such as magnesium (Mg), sodium (Na), lithium (Li), strontium (Sr), and barium (Ba) exhibit reversible carbonation and calcination reactions similar to calcium (Ca).

[0032] The exemplary embodiments of the invention presented in this patent application are not to be interpreted to pose limitations to the applicability of the appended claims. The verb "to comprise" is used in this patent application as an open limitation that does not exclude the existence of also unrecited features. The features recited in dependent claims are mutually freely combinable unless otherwise explicitly stated. The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims.Brief Description of Drawings

[0033] In the following, the invention will be described with reference to the accompanying exemplary, schematic drawings, in whichFigure 1 illustrates an apparatus for capturing CO2from gas containing CO2according to an embodiment of the invention,Figure 2 illustrates a fluidized bed mixing chamber according to another embodiment of the invention,Figure 3 illustrates a graph illustrating the effect of the present invention, andFigure 4 illustrates a fluidized bed mixing chamber according to another embodiment of the invention.Detailed Description of Drawings

[0034] Figure 1 depicts schematically an apparatus for capturing CO2from gas containing CO2according to an embodiment of the invention. The present invention is particularly useful in capturing CO2from exhaust gas which at some point will have changes or fluctuations in flow rate of CO2which need to be removed from the exhaust gas. A specific practical application of the present invention is to remove CO2 from exhaust gas from Electric Arc Furnace (EAF) and Argon Oxygen Decarburization (AOD) processes 300 which are interconnected stages in the production of high-quality steels. EAF is primarily used for melting scrap metal or direct reduced iron (DRI) to produce molten steel. The AOD process is used for refining molten steel to produce high-quality stainless steel and other alloys by precisely controlling the chemical composition and removing impurities. The EAF and AOD processes are used sequentially in stainless steel production. The EAF melts the raw materials, while the AOD refines the molten steel to achieve the desired quality and composition. Therefore, being a batch-like process an exhaust gas flow and CO2content of the exhaust gas flow changes during the run of the process. In EAF Stage CO2 levels are initially low, is increasing during oxidation of impurities. CO2content remains relatively high during the refining phase. In AOD initial oxygen blowing causes a spike in CO2levels, but as decarburization progresses and carbon content drops, CO2levels decrease. Final refining results in low CO2emissions in AOD.

[0035] Reactant material used in chemical looping, where a metal M is reversibly reacted between its carbonate form MCO3and its oxide form MO to separate carbon dioxide from other gases coming from either power generation or an industrial plant, forms a major part of a solid material used in the apparatus according to the invention.

[0036] Calcium Looping, CaL, is the most preferred method for capturing CO2using a chemical looping technique. The apparatus for capturing CO2from gas containing CO2comprises a circulating fluidized bed (CFB) carbonator reactor 100, and a circulating fluidized bed (CFB) calciner reactor 200. The FB carbonator reactor 100 and the FB calciner reactor are arranged to co-operate with each other in such a manner that they together form a calcium loop arrangement 10.

[0037] The fluidized bed carbonator reactor 100 comprises a carbonator chamber 102, in which the carbonation reaction of CaO takes place when gas containing CO2is fed into the carbonator chamber 102 at suitable conditions for carbonation reaction, which increases share of CaCO3and decreases CaO content in the solid material inventory. The carbonator reactor 100 comprises a windbox 104, to which a first inlet 106 for CO2containing gas is arranged. The windbox 104 is at the bottom of the carbonator chamber 102. The carbonator reactor 102 is provided with a fluidization grid 108 between the carbonator chamber 102 and the windbox 104, through which grid CO2containing gas is introduced into the carbonator chamber 102 as fluidization gas. In addition to the first inlet 106 at the bottom of the reactor, the carbonator chamber 102 is preferably provided with one or more secondary gas inlets 106’, 106” which opens into the reactor at a first and a second higher vertical level above the grid 108. This improves controlling of material circulation and material density distribution in the reactor 100. The present invention relates particularly to method of capturing CO2from gas containing CO2from Electric Arc Furnace (EAF) and the Argon Oxygen Decarburization (AOD) process, and it provides an advantageous apparatus for and method of taking care of strongly changing need for CO2reduction. Exhaust gas flow from EAF / AOD may, in addition to be fluctuating, it may even be discontinuous.

[0038] Since CaO is carbonized into CaCO3in the carbonator reactor 100, fresh or regenerated CaO needs to be brought into the reactor so that carbon capture capacity is maintained at desired level. The carbonator chamber 102 is providedat its lower part with a second inlet 110 for solid material which is in flow communication with the FB calciner reactor 200.

[0039] The carbonator chamber 102 is provided with a first outlet 112 for gassolid material suspension via which the gas containing solid material may be led to an at least one solid material separator 114, which is preferably a cyclone separator. The solid material separator 114 comprises an inlet 116 for gas-solid material suspension connected with the first outlet 112 for gas-solid material suspension in the carbonator chamber 102. The solid material separator 114 is further provided with a first outlet 118 for separated solid material, and a second outlet 120 for exhaust gas of reduced CO2concentration.

[0040] There is a first solid material return system 122 for separated solid material extending from the first outlet 118 for separated solid material to the carbonator chamber 102. The first solid material return system 122 comprises a loop seal 124 between the solid material separator 114 and the carbonator chamber 102 to prevent backflow of gas from the carbonator chamber to the separator and allow internal circulation of solid material in the system. The loop seal 124 is connected with the carbonator chamber 102 by a return duct 132, which opens into the carbonator chamber 102 through an inlet opening 130. There is also a solid material flow control system 126 in the first solid material return system, to which a first flow path 128 for solid material is connected. The first flow path 128 extends between the fluidized bed carbonator reactor 100 and the fluidized bed calciner reactor 200. The solid material at the outlet 118 of the solid material separator 114 has increased concentration of CaCO3compared to the concentration of CaCO3in the solid material which is introduced to the carbonator reactor 100. The solid material flow control system 126 provides a gas seal between the FB carbonator reactor 100 and the FB calciner reactor 200, which gas seal may be of similar construction to the loop seal 124. By means of the solid material flow control system 126 it is possible to control flow rate of solid material withdrawn from the carbonator reactor 100. The solid material, which is transferred from the carbonator reactor 100 to the first flow path 128 for solid material via the solid material flow control system 126 comprises solid material in which CaCO3has increased due to CO2capture in the carbonator reactor 100.

[0041] The fluidized bed calciner reactor 200 comprises a calciner chamber 202, which has a first inlet 204 for solid material to which the first flow path 128 forsolid material is connected. Thus, in more detail, the first flow path 128 for solid material extends between the solid material flow control system 126 of the carbonator reactor 100 and the first inlet 204 in the calciner chamber 202.

[0042] In the calciner chamber CaCO3is caused to split into CaO and CO2at prevailing conditions. When in use, a fuel is combusted in the calciner reactor 200 providing necessary heat for calcination reaction. Thus, there is a second inlet 206 for fuel material is arranged in the calciner chamber 202. The FB calciner chamber 202 further comprises an inlet for makeup material 207, preferably CaCO3. Suitable fuel material can be for example bio-residues and waste-derived fuels.

[0043] The calciner chamber 202 is provided with a first outlet 212 for gas-solid material suspension via which the gas containing solid material may be led to an at least one solid material separator 214, which is preferably a cyclone separator. The solid material separator 214 comprises an inlet 216 for gas-solid material suspension connected with the first outlet 212 for gas-solid material suspension in the calciner chamber 202. The solid material separator 214 is further provided with a first outlet 218 for separated solid material, and a second outlet 220 for exhaust gas, which substantially consist of CO2.

[0044] The calciner reactor 200 comprises a windbox 208, to which a third inlet of the reactor 210 for feeding oxygen containing gas is arranged. The windbox 210 is at the bottom of the calciner chamber 202. The calciner reactor 202 is provided with a fluidization grid 212 between the calciner chamber 202 and the windbox 208, through which grid oxygen containing gas is introduced into the carbonator chamber 202 as fluidization gas. The third inlet 210 for oxygen containing gas is in flow communication with a source of oxygen 214, such as an air separation unit, known as such. The third inlet 210 is also in controllable flow connection with an exhaust gas channel downstream the separator 214 by means of a gas recirculation channel 230. The gas recirculation channel 230 is provided with a control valve system 234 and a blower 232 for controlling the amount and / or proportion of recirculated gas in the fluidization gas. Thus, the fluidization gas in the calciner reactor 200 is mainly oxygen containing carbon dioxide. The recirculated gas is mainly CO2. In addition to the third inlet of the reactor 210 at the bottom of the reactor, the calciner reactor 200 is preferably provided with one or more secondary inlets 21 O’, 210” for oxygen containing gaswhich open into the reactor at a first and a second higher vertical level above the grid 212. This facilitates controlling of NOx emissions, particularly minimizing NOx emissions, in the exhaust gas resulted in oxy-combustion of fuel.

[0045] The solid separator 214 is provided with a second solid material return system 222 for separated solid material extending from the first outlet 218 for separated solid material to the calciner chamber 202 for returning a portion of the separated solid material back to the calciner chamber 202.

[0046] A second flow path 228 is arranged to the apparatus 10 extending between the fluidized bed calciner reactor 200 and the carbonator chamber 102 of the fluidized bed carbonator reactor 100 for introducing solid material to the carbonator reactor 100. Preferably the second flow path 228 is a plain solid material transfer channel which allows solid material transfer from the calciner reactor 200 to the carbonator chamber 100. There may be means for assisting transfer of solid material, such as fluidization means. The transfer channel is configured to only transfer the solid material but not accumulate or buffer the material or provide any heat or chemical treatment to the solid material. In other words, the solid material is in transfer movement in the channel, when in use. A first end, or the solids entry point, of the second flow path 228 is connected to the second solid material return system 222 of the calciner reactor 200, where the CaO concentration is at its maximum in the fluidized bed circulation in the calciner reactor 200. The second end the second flow path 228 is connected to the second inlet 110 of the carbonator reactor 100 for feeding regenerated reactant material in the carbonator chamber 102 CaO content of which has been increased in the calciner reactor 200. As it becomes clear from the description above, the fluidized bed carbonator reactor and a fluidized bed calciner reactor are arranged to cooperate with each other by the first flow path for solid material extending between the fluidized bed carbonator reactor and the calciner chamber of the fluidized bed calciner reactor, and the second flow path for solid material extending between the fluidized bed calciner reactor and the carbonator chamber of the fluidized bed carbonator reactor.

[0047] In the process solid material, calcium carbonate content of which has been increased in the carbonator reactor 100, is fed into the FB calciner reactor 200, where it is heated to a temperature that allows thermal decomposition into gaseous carbon dioxide and solid calcium oxide CaO, e.g. within a range of 850-950 °C. The solid material of which CaO content has been increased, is removed from the FB calciner reactor 200 and is fed into the FB carbonator reactor 100. The solid material is cooled, preferably to approximately 650 °C and is brought into contact with an exhaust gas containing CO2. The CaO and CO2react to form CaCO3, thus reducing the CO2concentration in the exhaust gas to a level suitable for emission to the atmosphere. Correspondingly the concentration of CaCO3is increased in the solid material when processed in the carbonator reactor. The solid material CaCO3of which has been increased is separated by the solid material separator 114 of the carbonator reactor contains also CaO and the proportions of the CaO and CaCO3is strongly affected by changes, or fluctuations in the CO2flow rate in the CO2containing exhaust gas, which can be caused by transient flow rate of exhaust gas and / or concentration of CO2 in the exhaust gas. Still, in order to reduce CO2content adequately the calciner reactor 200 needs to be capable of converting CaCO3into CaO with capacity which will suffice for peak circumstances also.

[0048] The first flow path 128 for solid material, which extends between the fluidized bed carbonator reactor 100 and the fluidized bed calciner reactor 200, comprises a fluidized bed mixing chamber 300, which is configured to even out transient changes in CaCO3concentration, such that a balanced state of concentration is reached over a predetermined period of residence time, which the solid material remains in the fluidized bed mixing chamber.

[0049] Now referring to the figure 2 which discloses the fluidized bed (FB) mixing chamber 300 arranged to the first flow path (128 in the figure 1) in more detailed manner. The FB mixing chamber is provided with an inlet for solid material 302 which is arranged in flow connection with the carbonator reactor 102, preferably with the first solid material return system 122 of the carbonator reactor. There is an inlet for fluidization gas 304 arranged to the bottom of the FB mixing chamber 300 and an outlet for solid material 308 arranged in flow connection with the calciner reactor 202. There is also an outlet for fluidization gas 310.

[0050] The FB mixing chamber 300, which may be of a rectangular horizontal cross section, comprises substantially vertical side walls 318, two of which are shown in the figure, a top wall 320, a wind box 314 and a fluidization grid 312 at its bottom part, to which wind box the inlet for fluidization gas 304 is arranged toopen. The top wall 320, the side walls 318 and the grid 312 define a cooler-free mixing space for a fluidized bed of solid material. The mixing chamber 300 may also have circular cross section, when there is only one continuous cylindrical side wall 318.

[0051] A fluidized bed of hot solid material is formed in the fluidized bed mixing chamber 300 arranged between the carbonator and calciner reactors. The fluidized bed mixing chamber comprises the grid 312 for feeding fluidizing gas to the chamber at a predetermined velocity through the wind box and nozzles of the grid 312. The gas is fed through the wind box such that the fluidizing velocity in the fluidized bed mixing chamber is relatively low, typically 0.1-1 .0 m / s. Thus, the bed in the fluidized bed mixing chamber is a so called bubbling fluidized bed, which has a well-defined upper surface 316. The FB mixing chamber has a cooler-free (except the fluidized bed of solid material, when in use) inner space, i.e. it does not include any heat exchange surfaces inside the chamber, also the side walls 318 forming the chamber are un-cooled. A detailed view A of the side wall 318 discloses an exemplary structure of the wall including a gas tight metal wall 3181 an inner refractory lining 3182 and an outer insulation 3183 on the surfaces of the wall 3181. As it can be seen, the walls 318 do not include any flow channels for heat transfer medium flow. This way the mixing process in the FB mixing chamber is an adiabatic mixing process.

[0052] The outlet for solid material 308 comprises a vertically extending discharge channel 322. The discharge channel 322 has an inlet opening 324 in the lower part of the mixing chamber, near the grid 312, and the outlet for solid material 308 is arranged at an upper part of the discharge channel 322. Direct flow connection from the outlet for solid material 308 to the outlet for fluidization gas 310 and the inlet 302 for solid material is prevented thus the inlet opening 324 is the only flow path between the mixing chamber and the outlet of solid material 308. The fluidization grid 312 extends horizontally inside the discharge channel, as a means to feed fluidizing gas to the lower part of the channel, such that the solid material may be maintained at fluidized state in the discharge channel, when the FB mixing chamber 300 is in operation i.e. simultaneously when the solid material in the mixing chamber is in fluidized stage. The fluidization gas which is fed to the discharge channel 322 can be controlled independently on themixing chamber. Thus, discharge channel 322 may have a separate wind box 314’.

[0053] The inlet for fluidization gas 304 is in flow connected with a source of fluidizing gas 306. The source of fluidizing gas is configured to provide pressurized gas in controlled manner to the wind box 314,314’. The source of fluidization gas selected so as to substantially exclude carbon oxides, particularly CO2from the fluidization gas. Suitable gas is for example nitrogen. This way a risk of forming deposits formed by the carbonation reaction into the FB mixing chamber. Also, not using CO2as a fluidization gas will prevent unnecessary carbonation reaction of CaO in in the mixing chamber. Also, the air may be used as fluidizing gas, that is a viable option, because CO2 concentration of the air is so small and the air is easily available gas.

[0054] The outlet for fluidization gas 310 is arranged at the top of the FB mixing chamber, above the outlet for solid material 308. The outlet for fluidization gas 310 leads the fluidization gas out from the FB mixing chamber, into the carbonator reactor chamber 102. Preferably the fluidization gas is introduced into the carbonator chamber 102 via a common inlet 130 with the solid material returned from the solid material separator 114, more precisely from the loop seal 124. Even if not shown in the figure 1 the fluidization gas outlet 310 may be alternatively or additionally connected with the second outlet 120 of the solid material separator 214. Since the outlet for solid material 308 is horizontally lower than the outlet for fluidization gas 310 and the inlet for solid material 302, height of surface of the fluidized bed in the mixing chamber 300 is maintained within desired range by applying fluidization gas continuously to the mixing chamber and the discharge channel. Flow of solid material through the outlet 308 takes place as overflow, the flow rate being ruled by difference of height (hydrostatic pressure) of the fluidized bed surface and height of the outlet from the grid level.

[0055] When in operation, mixed solid material flow to a substantially vertical discharge channel 322 through the opening 324 in the lower part of the mixing chamber. The fluidized material rises upwards in the discharge channel and exits to through the outlet opening 308 in the upper part of the channel 322. The velocity of the fluidizing gas in the discharge channel may vary in different operational situations, but in most cases, it is approximately the same or slightly higher than the velocity of the fluidizing gas in the chamber.

[0056] Volume of the mixing chamber 300 is dimensioned so that residence time of the solid material in the mixing chamber is at least 200 seconds and less than 1500s, preferably the residence time is 500-1000s.

[0057] The concentration of the CaCO3in material outlet flow follows an ideally mixed reactor behaviour i.e. dx_out / dt = (xjn - x_out) / Tau, whereTau= residence time x = concentration of CaCO3

[0058] As it becomes clear particularly from the description of figure 2, at least substantial decreasing of temperature of solid material which is transferred from the CFB carbonator reactor 100 to the CFB calciner reactor 200 via the first flow path 128 and the mixing chamber 300 is not desired to take place. Instead, solid material temperature is desired to remain the same as taken out from the carbonator reactor, or it may even beneficial to increase temperature of the solid material. In the figure 4 there shown an embodiment of the invention, according to which the first flow path 128 is provided with a heating device for preventing cooling of the solid material while it is on its transportation from the fluidized bed of solid material in the carbonator reactor 100, in the fluidized bed of solid material in the mixing chamber 300, removed from the fluidized bed mixing chamber 300 and fed to the calciner reactor 200. In the figure it shown that the first flow path of the apparatus 10 is provided with a heating device 326. The heating device may be an electric heater, like a resistive heater, or a heat exchanger having a heat transfer medium, such as steam or superheated steam, flowing inside the heat exchanger.

[0059] The heating device 326 is preferably arranged to the mixing chamber 300 because heat transfer in the fluidized bed is very efficient. As is shown in the figure 4 there may be alternatively or additionally a second heating device 328 in a flow channel which connects the mixing chamber 300 with the source of fluidizing gas 306. Furthermore, the apparatus may be provided alternatively or additionally with a third heating device 330 in a flow channel which connects the mixing chamber 300 with the calciner reactor 200. Still further, the apparatus may be provided alternatively or additionally to other heating means, with a fourth heating device 332 in a flow channel which connects the carbonator reactor 100 with the mixing chamber 300. Thus the first flow path 128 for solid materialextending between the fluidized bed carbonator reactor 100 and the calciner chamber 202 may be provided with one or more of the above mentioned heating devices 326, 328, 330, 332.

[0060] Figure 3 shows a graph illustrating the effect of the present invention in calcium looping process which shows time in its horizontal axis and CaCO3concentration in its vertical axis. Line 20 depicts CaCO3concentration in the solid material which enters the FB mixing chamber 300. As it can be seen the CaCO3concentration in the solid material which is to be fed to the FB calciner reactor 200 may have significant fluctuations when it is taken out from the carbonator reactor 100, which may be caused by fluctuations in flow rate of CO2containing gas and / or CO2concentration in the CO2containing gas. Line 21 depicts CaCO3concentration in the solid material which exits the FB mixing chamber and is conveyed to the calciner chamber 202 of the calciner reactor 200. The fluctuation has been effectively evened out by the FB mixing chamber according to the invention.

[0061] While the invention has been described herein by way of examples in connection with what are, at present, considered to be the most preferred embodiments, it is obvious to the skilled person that, along with the technical progress, the basic idea of the invention can be implemented in many ways. The invention and its embodiments are thus not limited to the examples and samples described above but they may vary within the contents of patent claims and their legal equivalents. The details mentioned in connection with any embodiment above may be used in connection with another embodiment when such combination is technically feasible.

Claims

Claims1. An apparatus for capturing CO2from gas containing CO2, comprising a fluidized bed carbonator reactor (100), and a fluidized bed calciner reactor (200) arranged to cooperate with each other, wherein a. the fluidized bed carbonator reactor (100) comprising i. a carbonator chamber (102) ii. a first inlet (106, 106’, 106”) for CO2containing gas in the carbonator chamber (102) at bottom of the fluidized bed carbonator reactor (100),Hi. a second inlet (110) for solid material in the carbonator chamber (102), iv. a first outlet (112) for gas-solid material suspension in the carbonator chamber (102), v. at least one solid material separator (114), having:

1. an inlet (116) for gas-solid material suspension connected with the first outlet (112) for gas-solid material suspension,2. a first outlet (118) for separated solid material, and3. a second outlet (120) for exhaust gas of reduced CO2concentration, vi. a first solid material return system (122) for separated solid material extending from the first outlet (118) for separated solid material to the carbonator chamber (102), b. the fluidized bed calciner reactor (200) comprising i. a calciner chamber (202) ii. a first inlet (204) for solid material in the calciner chamber (202),Hi. a second inlet (206) for fuel material in the calciner chamber, iv. a third inlet (210,21 O’, 210”) for oxygen containing gas in the calciner chamber (202), v. a first outlet (212) for gas-solid material suspension in the calciner chamber (202). vi. at least one solid material separator (214), having:

1. an inlet (216) for gas-solid material suspension connected with the fist outlet (212) for gas-solid material suspension,2. a first outlet (218) for separated solid material, and3. a second (220) outlet for CO2enriched gas, vii. a second solid material return system (222) for separated solid material extending from the first outlet (215) for separated solid material to the calciner chamber (202), and the apparatus further comprising c. a first flow path (128) for solid material extending between the fluidized bed carbonator reactor (100) and the calciner chamber (202) of the fluidized bed calciner reactor (200), d. a second flow path (228) for solid material extending between the fluidized bed calciner reactor (200) and the carbonator chamber (102) of the fluidized bed carbonator reactor (100), characterized in that e. the first flow path (128) comprises a fluidized bed mixing chamber (300), comprising i. inlet (302) for solid material arranged in flow connection with the carbonator reactor (100), ii. inlet (304) for fluidization gas,Hi. outlet (308) for solid material arranged in flow connection with the calciner reactor (200), iv. outlet (310) for fluidization gas.

2. An apparatus according to claim 1 , characterized in that the outlet (310) for fluidization gas in the fluidized bed mixing chamber (300) is arranged in flow connection with the carbonator reactor (100).

3. An apparatus according to claim 2, characterized in that the outlet (310) for fluidization gas is connected to the carbonator chamber (102).

4. An apparatus according to claim 1 , characterized in that the second flow path (228) consists of a solid material transfer channel extending from the second solid material return system to the carbonator reactor (100).

5. An apparatus according to anyone of the preceding claims, characterized in that the fluidized bed mixing chamber (300) has a mixing space, which mixing space is a cooler-free space.

6. An apparatus according to claim 1 or 5, characterized in that the fluidized bed mixing chamber (300) comprises substantially vertical side walls (318), a top wall (320), a wind box (314) and a fluidization grid (312) at its bottom part, to which wind box (314) the inlet (304) for fluidization gas is arranged to open, and in that the top wall (320), the side walls (318) and the grid (312) of the fluidized bed mixing chamber (300) are of uncooled structure so as to provide adiabatic mixing of reactant material.

7. An apparatus according to anyone of the preceding claims 1-4, characterized in that the first flow path (128) is provided with a heating device (326) for preventing decreasing of temperature of the solid material.

8. An apparatus according to claim 6, characterized in that the top wall (320) and the side walls (318) of the fluidized bed mixing chamber are provided with a heat insulation (3182,3183).

9. An apparatus according to claim 8, characterized in that the heat insulation comprises a refractory lining (3182) arranged inside the fluidized bed mixing chamber (300).

10. An apparatus according to claim 1 , 5 or 6, characterized in that the outlet (308) for solid material in the mixing chamber (300) comprises a vertically extending discharge channel (322) having an opening (324) in the lower part of the mixing chamber (300), wherein the outlet (308) is arranged at an upper part of the discharge channel (322), wherein the discharge channel is provided with means (314’) to feed fluidizing gas to the lower part of the discharge channel (322).

11. A method of operating an apparatus for capturing CO2from gas containing CO2according to claiml , the method comprising a. introducing CO2containing gas into the carbonator chamber (102) via the first inlet (106, 106’, 106”) for CO2containing gas, b. feeding solid material into the carbonator chamber via the second inlet (110) for solid material,c. arranging a fluidized bed of solid material in the carbonator chamber (102), d. causing CO2in the gas to react with solid CaO in the fluidized bed of solid material, forming CaCO3and decreasing CaO content of the solid material, and increasing CaCO3 content of the solid material, e. removing the solid material from the fluidized bed of solid material in the carbonator reactor (100), and feeding the removed solid material to the fluidized bed mixing chamber (300), f. arranging a fluidized bed of solid material in the mixing chamber (300) by feeding fluidization gas into the mixing chamber, g. removing solid material from the fluidized bed mixing chamber (300) and h. feeding the removed solid material to the calciner reactor (200).

12. A method according to claim 11 , characterized in that residence time of the solid material in the fluidized bed mixing chamber (300) is at least 200 seconds and less than 1500 seconds, preferably the residence time is 500 - 1000 seconds.

13. A method according to claim 11 , characterized in that fluidization gas is led from the mixing chamber (300) to the carbonator reactor (100).

14. A method according to claim 11 , characterized in that solid material is fed directly from the calciner reactor (200) into the carbonator chamber (102).

15. A method according to claim 11 , characterized in that decrease of temperature of the solid material during the step e. to g. is prevented by bringing heat to the solid material.

16. A method according to anyone of the preceding claims 11 -15, characterized in that fluidization gas fed to the mixing chamber (300) is a gas excluding CO2.

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